Heating resistor flow rate measuring instrument

ABSTRACT

A heating resistor flow rate measuring instrument, having a heating resistor and a thermally sensitive resistor disposed in a sub-passage of a main passage. The heating resistor is disposed in a portion of the sub-passage which has an upstream-side opening substantially perpendicular to the forward direction of a fluid flow and a downstream-side opening substantially parallel to the reverse flaw direction, and the thermally sensitive resistor is disposed in a second sub-passage portion having an upstream-side opening substantially parallel to the forward flow direction and a downstream-side opening substantially perpendicular to the reverse flow direction. Based on changes in resistor temperatures when the flow direction reverses, a precise flow rate may be determined even when a pulsation occurs in the fluid, as both a plus effect caused by a backward flow is eliminated and a value corresponding to the backward flow may be subtracted to obtain accurate flow information.

TECHNICAL FIELD

The present invention relates to a heating resistor flow rate measuringinstrument for measuring the flow rate of a fluid when the fluid flowingin the forward direction may cause a pulsation accompanying a backwardflow, and more particularly to a heating resistor flow rate measuringinstrument suitable for measuring the flow rate of intake air of anautomobile engine.

BACKGROUND ART

JP,B 4-34686, for example, discloses one of prior-art heating resistorflow rate measuring instruments.

Also, JP,A 8-159833, for example, discloses one of heating resistor flowrate measuring instruments capable of measuring the flow rate of a fluidwhen the fluid flowing in the forward direction may cause a pulsationaccompanying a backward flow.

In the flow rate measuring instrument disclosed in JP,A 8-159833, asub-passage is provided in a main passage, and a heating resistor isdisposed in the sub-passage. The sub-passage includes a pair ofsub-passage portions that are extended substantially parallel to acenter axis of the main passage and are opened in directions opposite toeach other. An end of one sub-passage portion positioned away from theopening side has a communicating portion communicated with the vicinityof an opening of the other sub-passage portion. The heating resistor isdisposed in each sub-passage between the two communication portions.

DISCLOSURE OF INVENTION

In the prior-art flow rate measuring instrument disclosed in theabove-cited JP,B 4-34686, however, when a backward pulsating flow occursin the fluid flowing in the forward direction, it is difficult tocompletely eliminate the effect of the backward pulsating flow and toprecisely measure the flow rate of the forward flow.

Also, in the flow rate measuring instrument disclosed in the above-citedJP,A 8-159833, the effect of the backward pulsating flow can beeliminated to some extent, but a plurality of heating resistors and atleast one thermally sensitive resistor are required, thus resulting in aproblem that the circuit configuration is complicated.

The present invention has been made in view of the above-mentionedproblems in the prior art, and its object is to provide a heatingresistor flow rate measuring instrument which has a simplified circuitconfiguration and is able to precisely detect the flow rate in theforward direction even for a fluid that may cause a pulsationaccompanying a backward flow, such as intake air of an automobileengine.

Particularly, an object of the present invention is to provide an airflow rate measuring instrument in which, in measurement of the flow rateof intake air of an automobile engine, a large plus error resulting whena pulsation accompanying a backward flow occurs near a throttlefully-opened stroke in a specific range of revolution speed can beeliminated and fuel control, etc. precisely responsive to operationconditions can be achieved.

To achieve the above objects, the present invention is constituted asfollows.

-   (1) A heating resistor flow rate measuring instrument comprising a    heating resistor and a thermally sensitive resistor both disposed in    a main passage, and measuring the flow rate of a fluid passing    through the main passage, the instrument including a first location    exposed to the fluid flowing in one direction within the main    passage in a larger amount than the fluid flowing in a direction    opposite to the one direction; and a second location exposed to the    fluid flowing in the opposite direction within the main passage in a    larger amount than the fluid flowing in the one direction, wherein    the heating resistor is disposed at the first location, the    thermally sensitive resistor is disposed at the second location, and    the flow rate of the fluid passing through the main passage is    measured based on amounts of heat radiated from the heating resistor    and the thermally sensitive resistor.-   (2) In above (1), preferably, the heating resistor is heated to be    higher than a fluid temperature in the main passage by a first    predetermined temperature, and the thermally sensitive resistor is    heated to be higher than the fluid temperature in the main passage    by a second predetermined temperature.-   (3) In above (1) or (2), preferably, the first location is provided    by a first sub-passage having a first opening that faces    substantially perpendicular to the fluid flowing in the one    direction, and a second opening that faces substantially parallel to    the fluid flowing in the opposite direction, and the second location    is provided by a second sub-passage having a third opening that    faces substantially perpendicular to the fluid flowing in the    opposite direction, and a third opening that faces substantially    parallel to the fluid flowing in the one direction.-   (4) In above (1) or (2), preferably, the first location is provided    by a first sub-passage having a first opening that faces    substantially perpendicular to the fluid flowing in the one    direction, and a second opening that faces substantially parallel to    the fluid flowing in the opposite direction, and a wall portion    having a surface substantially perpendicular to the lengthwise    direction of the main passage is formed in the one-direction side of    the second location.-   (5) In above (1) or (2), preferably, the first location is provided    by a first sub-passage having a first opening that faces    substantially perpendicular to the fluid flowing in the one    direction, and a second opening that faces substantially    perpendicular to the fluid flowing in the opposite direction and has    a smaller opening area than the first opening, and the second    location is provided by a second sub-passage having a third opening    that faces substantially perpendicular to the fluid flowing in the    one direction, and a fourth opening that faces substantially    perpendicular to the fluid flowing in the opposite direction and has    a larger opening area than the third opening.-   (6) In above (1), (2), (3), (4) and (5), preferably, the thermally    sensitive resistor is heated to a temperature 20° C.–40° C. higher    than the fluid temperature in the main passage.-   (7) A heating resistor flow rate measuring instrument for measuring    the flow rate of a fluid passing through a passage, the instrument    comprising a first heating resistor radiating a larger amount of    heat to the fluid flowing in one direction within the passage than    to the fluid flowing in a direction opposite to the one direction;    and a second heating resistor radiating a larger amount of heat to    the fluid flowing in the opposite direction than to the fluid    flowing in the one direction; wherein a bridge circuit including the    first heating resistor and the second heating resistor is formed,    the first heating resistor is heated to be higher than the second    heating resistor by a certain temperature, and the flow rate of the    fluid passing through the passage is measured based on the amount of    heat radiated from the first heating resistor.-   (8) A heating resistor flow rate measuring instrument comprising a    heating resistor and a thermally sensitive resistor both disposed in    a main passage, and measuring the flow rate of a fluid passing    through the main passage, wherein the instrument includes a    thermally-sensitive-resistor arrangement location in which the    thermally sensitive resistor is disposed and radiates a larger    amount of heat when the fluid flows in a direction opposite to one    direction within the main passage than when the fluid flows in the    one direction; the heating resistor is heated to be higher than a    fluid temperature in the passage by a first predetermined    temperature; the thermally sensitive resistor is heated to be higher    than the fluid temperature in the passage by a second predetermined    temperature; and the flow rate of the fluid passing through the    passage is measured based on amounts of heat radiated from the    heating resistor and the thermally sensitive resistor.-   (9) In above (8), preferably, the heating resistor is disposed at a    location in which the heating resistor radiates a larger amount of    heat when the fluid flows in the one direction within the main    passage than when the fluid flows in the opposite direction.

With the present invention, since the passage is constructed such thatthe thermally sensitive resistor is more easily exposed to a backwardflow and the heating resistor is harder to be exposed to the backwardflow, the flow rate in the forward direction can be precisely detectedeven for a fluid that may cause a pulsation accompanying the backwardflow, such as intake air of an automobile engine.

Further, the number of resistors used is small and hence the instrumentcan be realized with a simple circuit configuration.

Particularly, in measurement of the flow rate of intake air of anautomobile engine, a large plus error resulting when a pulsationaccompanying a backward flow occurs near a throttle fully-opened strokein a specific range of revolution speed can be eliminated and fuelcontrol precisely responsive to operation conditions can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the basic principle of a heating resistorflow rate measuring instrument of the present invention.

FIG. 2 is a graph showing the relationship between an air pressure in anintake manifold, which is a pressure upstream of an engine cylinder, andan output of an air flow rate measuring instrument resulting when theengine revolution speed is held constant.

FIG. 3 is a graph showing the relationship between a plus error, whichis caused in a prior-art heating resistor flow rate measuring instrumentupon the generation of a pulsation accompanying a backward flow near athrottle fully-opened stroke, and the flow rate detected by the flowrate measuring instrument.

FIG. 4 is a graph showing an output of a bypassing flow rate measuringinstrument resulting when a pulsation accompanying a backward flowoccurs.

FIG. 5 is a graph showing the relationship between a plus error causedupon the generation of a pulsation accompanying a backward flow near athrottle fully-opened stroke and an output of the flow rate measuringinstrument shown in FIG. 4.

FIG. 6 is a graph showing the relationship between a plus error, whichis caused in the heating resistor flow rate measuring instrument of thepresent invention upon the generation of a pulsation accompanying abackward flow near a throttle fully-opened stroke, and an output of theflow rate measuring instrument.

FIG. 7 is a sectional view of a heating resistor flow rate measuringinstrument according to a first embodiment of the present invention.

FIG. 8 is a sectional view taken along the line A—A in FIG. 7.

FIG. 9 is a sectional view taken along the line B—B in FIG. 7.

FIG. 10 is a sectional view of a sub-passage in a heating resistor flowrate measuring instrument according to a second embodiment of thepresent invention.

FIG. 11 is a sectional view of a sub-passage in a heating resistor flowrate measuring instrument according to a third embodiment of the presentinvention.

FIG. 12 is a schematic sectional view of a heating resistor flow ratemeasuring instrument according to a fourth embodiment of the presentinvention.

FIG. 13 is an external appearance view, looking from the upstream side,of the heating resistor flow rate measuring instrument according to thefourth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Practical embodiments of a heating resistor flow rate measuringinstrument of the present invention will be described below withreference to the drawings.

FIG. 1 is a diagram showing the basic principle of a heating resistorflow rate measuring instrument of the present invention. In FIG. 1, itis assumed that a fluid flow directing rightward from the left side is aforward flow 11, and a fluid flow directing leftward from the right sideis a backward flow 12.

The heating resistor flow rate measuring instrument of the presentinvention includes a sub-passage 3 disposed in a main passage 4. Aheating resistor 1 (Rh) and a thermally sensitive resistor 2 (Rc) aredisposed in the sub-passage 3.

The sub-passage 3 comprises a first sub-passage having an upstream-sideopening face substantially perpendicular to the forward direction of thefluid flow and a downstream-side opening face substantially parallel tothe direction opposite to the forward fluid flow, and a secondsub-passage having an upstream-side opening face substantially parallelto the forward direction of the fluid flow and a downstream-side openingface substantially perpendicular to the direction opposite to theforward fluid flow.

The heating resistor 1 is disposed in the first sub-passage, and thethermally sensitive resistor 2 is disposed in the second sub-passage.

Thus, the first sub-passage including the heating resistor 1 disposedtherein is constructed such that the fluid flowing in the backwarddirection is harder to flow into the first sub-passage than the fluidflowing in the forward direction. The second sub-passage including thethermally sensitive resistor 2 disposed therein is constructed such thatthe fluid flowing in the forward direction is harder to flow into thesecond sub-passage than the fluid flowing in the backward direction.

The heating resistor 1 is heated to a temperature about 100° C.–300° C.higher than a fluid temperature Ta in the first sub-passage, andprovides an output depending on the flow rate of the fluid in accordancewith the amount of heat radiated to the fluid.

Although the thermally sensitive resistor 2 is used in the prior art todetect the fluid temperature without being heated, the thermallysensitive resistor 2 in the present invention is heated to be higherthan the fluid temperature Ta by about 1/10 to ½ of the temperature towhich the heating resistor 1 is heated.

For example, the heating resistor 1 is heated to the fluid temperatureTa+200° C. (in fact, a temperature Tc of the thermally sensitiveresistor 2 +160° C.), and the thermally sensitive resistor 2 is heatedto the fluid temperature Ta+40° C.

With the construction described above, when the forward flow 11generates in the main passage 4, the fluid flowing in the forwarddirection flows into the first sub-passage and the heating resistor 1 iscooled.

On the other hand, the forward flow 11 flows into the second sub-passagein a less amount and develops a small cooling action upon the thermallysensitive resistor 2. Also, since the thermally sensitive resistor 2 isheated to be 40° C. higher than the fluid temperature in the secondsub-passage, the thermally sensitive resistor 2 is not heated by theheat radiated from the heating resistor 1. Accordingly, the heatingresistor 1 is held in the same state as when it is heated to Ta+200° C.

Conversely, when the backward flow 12 generates in the main passage 4,the backward flow 12 acts upon the second sub-passage and the thermallysensitive resistor 2 is cooled. Although the thermally sensitiveresistor 2 is heated in advance to be 40° C. higher than the fluidtemperature in the second sub-passage, the temperature of the thermallysensitive resistor 2 lowers under cooling due to the backward flow.

The backward flow 12 flows into the first sub-passage in a less amountand develops a small cooling action upon the heating resistor 1.

In that case, the temperature of the thermally sensitive resistor 2lowers from the temperature to which it has been heated so far, andshows a value close to Ta. Also, the temperature of the heating resistor1 lowers and shows a value close to Ta+160° C. Therefore, the amount ofheat radiated to the fluid is decreased, and the output of the heatingresistor 1 is reduced. As a result, a plus error caused upon thegeneration of the backward flow is canceled as described later.

In the present invention, as described above, the thermally sensitiveresistor 2 is heated in advance to be higher than the fluid temperaturein the sub-passage, and the heating resistor 1 and the thermallysensitive resistor 2 are arranged such that the cooling effect upon themdiffers from each other between when the forward flow 11 flows in themain passage 4 and when the backward flow 12 flows in the main passage4.

With those features, since the heating resistor 1 has a high detectionsensitivity for the forward flow 11 in the main passage 4 and a lowdetection sensitivity for the backward flow 12 in the main passage 4,the later-described plus error caused upon the generation of thebackward flow is canceled and the flow rate in the forward direction canbe precisely detected even for a flow that may cause a pulsationaccompanying a backward flow, such as intake air of an automobileengine.

A measurement error caused by a pulsation in a prior-art flow ratemeasuring instrument will now be described with reference to FIG. 2,taking as an example measurement of the flow rate of intake air in theautomobile engine.

FIG. 2 is a graph showing the relationship between an air pressure in anintake manifold (hereinafter referred to as an “in-manifold pressure”),which is a pressure upstream of an engine cylinder, and an output of anair flow rate measuring instrument resulting when the engine revolutionspeed, i.e., the pulsation period, is held constant.

In FIG. 2, the in-manifold pressure changes depending on the openingdegree of a throttle valve (throttle opening), which controls the flowrate of the intake air in interlock with an accelerator pedal.

The air flow rate measuring instrument is usually disposed upstream ofthe throttle valve. When the throttle opening is small, an intakepulsation is small due to the contraction effect of an intake pipe, anda measurement error caused by the pulsation does not occur in the airflow rate measuring instrument. Therefore, an output of the air flowrate measuring instrument increases monotonously depending on thein-manifold pressure.

However, the intake pulsation increases with an increase of the throttleopening. In a pulsation range where a minimum value of the pulsationamplitude approaches 0, the output of the air flow rate measuringinstrument is reduced in spite of the true intake flow rate increasing,thus resulting in a minus error.

Such a phenomenon is called a two-value phenomenon because the output ofone air flow rate measuring instrument has two different flow ratevalues from each other.

The two-value phenomenon is primarily attributable to that therelationship of flow rate versus output of the heating resistor air flowrate measuring instrument is non-linear and the heating resistor airflow rate measuring instrument has a response delay. The techniquecalled a bypassing method is known as a measure for coping with thetwo-value phenomenon.

According to the bypassing method, a heating resistor is disposed in abypass passage having a bent flow path formed therein so that themeasured value is shifted toward the plus side based on the bypassinginertia effect upon the generation of a pulsation, thereby canceling theminus error. That technique is known and therefore a detailed describedthereof is omitted here.

The present invention provides a technique capable of not onlymaintaining the above-mentioned bypassing effect, but also coping withthe two-value phenomenon in more satisfactory manner.

Furthermore, near a throttle fully-opened stroke in a specific range ofrevolution speed, there occurs a pulsation in which a minimum value ofthe pulsation amplitude becomes below 0, i.e., a pulsation accompanyinga backward flow. At that time, the output of the air flow rate measuringinstrument jumps up to a large extent, thus resulting in a large pluserror.

Such a plus error caused by the backward flow will be described withreference to FIGS. 3 to 5.

FIG. 3 is a graph showing the relationship between a jumping-up (pluserror), which is caused in a prior-art heating resistor flow ratemeasuring instrument upon the generation of a pulsation accompanying abackward flow near a throttle fully-opened stroke, and an output of theflow rate measuring instrument, the graph being expressed as a waveformon an assumption that there is no response delay.

In FIG. 3, the heating resistor cannot discriminate the direction of thefluid flow and hence detects the backward flow as the forward flow inspite of that the actual backward flow should be detected as a minusvalue, as indicated by a dotted line. Accordingly, when the backwardflow occurs, the average value (integrated value) of an output of theheating resistor includes a large positive error relative to the truevalue.

FIG. 4 is a graph showing an output of the so-called bypassing flow ratemeasuring instrument in which the heating resistor is less exposed tothe backward flow by providing a sub-passage having a bent portion, forexample, with intent to reduce the plus error, shown in FIG. 3, causedin the prior-art heating resistor air flow rate measuring instrument,the output being resulted when a pulsation accompanying a backward flowoccurs, on an assumption that there is no response delay as in the graphof FIG. 3.

As seen from FIG. 4, optimization of the bypass passage can realize astructure in which, even when the backward flow generates in the mainpassage, the backward flow occurred in the bypass passage where theheating resistor is disposed can be held down very small. With thatstructure, it is possible to avoid the plus count of a minus value,which is caused because the heating resistor cannot detect whether thedirection of the fluid flow is forward or backward.

However, optimization of the bypass passage cannot completely cancel theplus error caused by the backward flow. The reason is that while thetrue flow rate is given as the average value (integrated value)including the minus value resulting from the backward flow, thebypassing method provides the average value (integrated value) largerthan the true value because the backward flow is just cut off withoutincluding the minus value in the average value (integrated value).

In other words, unless the output resulting from the forward flow isdeduced from a total flow rate depending on the backward flow asindicated by a dotted line, the true flow rate and the output averagevalue do not match with each other.

FIG. 5 is a graph showing an actual output of the heating resistor flowrate measuring instrument as compared with the graph of FIG. 4 in whichthere is no response delay. The actual output of the heating resistorflow rate measuring instrument has a less-sharpened waveform, i.e., awaveform having a gentler slope than that of the true output, due to theresponse delay in the outputting.

In the actual heating resistor flow rate measuring instrument causingthe response delay, as shown in FIG. 5, the plus error caused by thebackward flow is reduced because of the two-value phenomenon describedabove as the known phenomenon, but it is not completely canceled.

The present invention is able to further compensate for the measurementerror caused by the backward flow, which is not completely canceled bythe bypassing method, while maintaining the bypassing effect.

FIG. 6 is a graph showing the relationship between a jumping-up (pluserror), which is caused in the heating resistor flow rate measuringinstrument of the present invention upon the generation of a pulsationaccompanying the backward flow near a throttle fully-opened stroke, andan output of the flow rate measuring instrument.

In the present invention, the heating resistor 1 is disposed in thebypass passage described above so as to suppress the two-valuephenomenon based on the bypassing effect and to reduce the plus errorcaused by the backward flow. Further, in the present invention, asdescribed above with reference to FIG. 1, the thermally sensitiveresistor 2 is disposed to be more easily exposed to the backward flowthan to the forward flow, and it is heated in advance to a level higherthan the fluid temperature in the sub-passage by the predeterminedtemperature. With those features, when the backward flow 12 generates inthe main passage 4, the temperature to which the heating resistor 1 isheated becomes lower, whereby the plus error can be further reducedwhich is caused in the heating resistor air flow rate measuringinstrument upon the generation of a pulsation accompanying the backwardflow.

A dotted line in FIG. 6 represents the output of the heating resistorair flow rate measuring instrument resulting when the influence of thebackward flow is reduced by the optimization of the bypassing effectdescribed above, while a solid line represents the output of the heatingresistor air flow rate measuring instrument of the present invention.

With the present invention, for the fluid flowing only as the forwardflow without including the backward flow, the flow rate measuringinstrument produces the same output as that obtained with the bypassingmethod. Upon the generation of a pulsation accompanying the backwardflow, however, the thermally sensitive resistor 2 is cooled by thebackward flow, whereupon the temperature to which the heating resistor 1is heated becomes lower and the amount of heat radiated from the heatingresistor 1 is reduced. Accordingly, the output of the heating resistorair flow rate measuring instrument is reduced.

In practice, since the temperature of the thermally sensitive resistor 2is maintained in a lowered state due to the response delay in the statesof not only the backward flow, but also the forward flow, the instrumentproduces the output shifting toward the minus direction as a whole.

Thus, the plus error caused by the backward flow is reduced and the flowrate can be precisely detected even for a flow that may cause apulsation accompanying the backward flow, such as intake air of anautomobile engine.

A heating resistor air flow rate measuring instrument as a practicalembodiment of the present invention will be described below withreference to FIGS. 7 to 13.

FIG. 7 is a sectional view showing a state in which a heating resistorflow rate measuring instrument according to a first embodiment of thepresent invention is mounted as a module in the main passage 4. FIG. 8is a sectional view taken along the line A—A in FIG. 7, and FIG. 9 is asectional view taken along the line B—B in FIG. 7.

Referring to FIGS. 7 to 9, the heating resistor 1 and the thermallysensitive resistor 2 are disposed respectively in a first bypass 302 anda sub-passage (for arrangement of the thermally sensitive resistor) 306of a bypass 3 while being fixed to corresponding terminals 14, and theyare electrically connected to an electronic circuit 5 via metal wires15.

A housing 6 for protectively containing the electronic circuit 5 is aplastic-molded part formed by insert molding with the terminals 14 andconnector terminals 9 incorporated therein as metal terminals.

The housing 6 comprises the bypass 3 constituting a bent flow path inwhich the heating resistor 1 is disposed, a passage portion partlyconstituting a sub-passage 306 in which the thermally sensitive resistor2 is disposed, a case portion forming a frame in which the electroniccircuit 5 is mounted in a protective way, a connector portion in whichthe connector terminals 9 are disposed for electrical connection to anexternal device, and a flange portion used for fixing the flow ratemeasuring instrument to a member 16 that constitutes a main passage 4described later, These components are formed integrally with each other.

The housing 6 and the electronic circuit 5 are fixedly bonded to a metalbase 7, and the bypass 3 and the sub-passage 306 are completed byjoining of a bypass cover 13 and a circuit cover 10 in respective placessuch that the electronic circuit 5 is protected at its surroundings.Thus, the flow rate measuring instrument is constructed as a modulecontaining the circuit, the sensors, the sub-passage, the connector,etc. in an integral structure.

The main passage 4 is a flow path through which a fluid to be measuredflows, and it corresponds to an intake pipe extending from an aircleaner to a position upstream of an engine cylinder, for example, whenthe present invention is applied to an automobile engine.

In a heating resistor air flow rate measuring instrument for use in anautomobile, the member 16 constituting the main passage 4 is constitutedas a body dedicated to the heating resistor air flow rate measuringinstrument and connected to midway the intake pipe, or it is constitutedby employing an air cleaner, a duct, a throttle body or the like incommon.

An insertion hole 17 is formed in a wall of the main passageconstituting member 16, and a measuring unit containing both the heatingresistor 1 and the thermally sensitive resistor 2 mounted therein isinserted through the insertion hole 17 so as to position in the mainpassage 4. By fixing the housing 6 to the main passage constitutingmember 16, the instrument is set in a state capable of measuring theflow rate of air flowing through the main passage.

The bypass 3 is formed as a roundabout path made up of a bypass inlet301 opened in a plane substantially perpendicular to a center axis ofthe main passage 4, a first bypass 302 extending substantially parallelto the center axis of the main passage 4, a roundabout portion 303 forreversing 180° the flow direction at a downstream end of the firstbypass 302, a second bypass 304 extending substantially parallel to thefirst bypass in the opposite direction, and a bypass outlet 305 openedat a downstream end of the second bypass 304 in a plane substantiallyperpendicular to the center axis of the main passage 4.

The heating resistor 1 is disposed, as described above, in the firstbypass 302 of the roundabout bypass 3. Thus, a structure is obtained inwhich, when the backward flow 12 generates in the main passage 4, thebackward flow is hard to flow in up to the location where the heatingresistor 1 is disposed.

The sub-passage 306 is formed as a straight tubular path having asub-passage inlet 401 opened in a plane substantially perpendicular tothe center axis of the main passage 4, and a sub-passage outlet 402formed downstream of the sub-passage inlet.

A projection-shaped flow restrictor 403 is formed in the tubular path ofthe sub-passage 306, and the thermally sensitive resistor 2 is disposedin a position shielded by the flow restrictor 403 when viewed from theupstream side.

Accordingly, when the forward flow 11 flows in the main passage 4, thepresence of the flow restrictor 403 forms a region in which the fluidflow turns to a separated flow at a location where the thermallysensitive resistor 2 is disposed. Hence, the separated flow has a verylow flow speed as compared with that of a flow reaching the locationwhere the thermally sensitive resistor 2 is disposed when the backwardflow 12 flows in the main passage 4.

The electronic circuit 5 is constituted as containing a circuit shown inFIG. 1 and has a bridge circuit including the heating resistor 1 (Rh)and the thermally sensitive resistor 2 (Rc). More specifically, one endof the heating resistor 1 is connected to the other end of the heatingresistor 1 through resistors R1, R2, the thermally sensitive resistor 2,and a resistor R3.

Further, the junction between the resistors R1 and R2 is grounded, andthe junction between the thermally sensitive resistor 2 and the resistorR2 is connected to one input terminal of an operational amplifier OP1.An output terminal of the operational amplifier OP1 is connected to thebase of a transistor Tr, and the emitter of the transistor Tr isconnected to the junction between the resistor R3 and the heatingresistor 1.

In addition, the junction between the thermally sensitive resistor 2 andthe resistor R1 is connected to the other input terminal of theoperational amplifier OP1.

The junction between the thermally sensitive resistor 2 and the resistorR1 is also connected to one input terminal of an operational amplifierOP2, and further connected to an output terminal of the operationalamplifier OP2 through a resistor R4.

The other input terminal of the operational amplifier OP2 is connectedto a reference voltage source Vref through a resistor R5 and is alsogrounded through a resistor R6.

After balancing the bridge circuit including the heating resistor 1 andthe thermally sensitive resistor 2, electric currents Ih and Ic flowingrespectively through the heating resistor 1 and the thermally sensitiveresistor 2 are adjusted so that the heating resistor 1 is sufficientlyheated (for example, to the fluid temperature +200° C., i.e., thetemperature of the thermally sensitive resistor +160° C.) and thethermally sensitive resistor 2 is slightly heated (for example, to thefluid temperature +40° C.).

With the construction described above, the flow rate can always beprecisely measured even for a flow that may change from a steady flow toa pulsating flow and further to a pulsating flow accompanying a backwardflow, such as intake air of an automobile engine.

Stated another way, when a pulsating flow generates and a pulsationincreases to such an extent that a minimum flow speed becomes close to0, the minus error called the two-value phenomenon occurs as describedabove. By arranging the heating resistor 1 in the roundabout bypass 3,however, the minus error can be suppressed and canceled with thebypassing inertia effect mentioned above.

Further, when the pulsation amplitude increases and generates apulsating flow accompanying a backward flow, the structure of theroundabout bypass 3 serves to suppress the backward flow from flowing inup to the location where the heating resistor 1 is disposed.

Those advantages are also obtained with the prior art, but a reductionof the plus error caused by the backward flow is not yet sufficient, asdescribed above.

With the first embodiment of the present invention, since the thermallysensitive resistor 2 slightly heated is disposed in the sub-passage 306such that the thermally sensitive resistor 2 is harder to be exposed tothe forward flow and is more easily exposed to the backward flow, thethermally sensitive resistor 2 is cooled by the fluid upon thegeneration of the backward flow and the temperature of the heatingresistor 1, which is controlled to be held at a level higher than thatof the thermally sensitive resistor 2 by the predetermined temperature,is reduced.

Accordingly, the amount of heat radiated from the heating resistor 1 tothe fluid is also reduced and so is an electric current supplied for theheating. In other words, the measured value of the flow rate is shiftedtoward the minus side upon detection of the backward flow, whereby theplus error caused by the backward flow is canceled and the measurementcan be made at higher accuracy.

In practice, the thermally sensitive resistor 2 is also cooled by theforward flow depending on the flow rate, but the measurement accuracy inthe state of the forward flow can be maintained by measuring therelationship between the flow rate and the output in the state of theforward flow beforehand and obtaining an output characteristic of theheating resistor air flow rate measuring instrument based on themeasured relationship.

Thus, the highly precise measurement can be achieved not only in thestate of the forward flow, but also when a pulsation accompanying thebackward flow occurs.

FIG. 10 is a schematic sectional view of a sub-passage 306 in a heatingresistor flow rate measuring instrument according to a second embodimentof the present invention. The other construction than the sub-passage isthe same as that of the above-described first embodiment, and henceillustration and a detailed description thereof are omitted here.

In the second embodiment of the present invention, the configuration ofthe sub-passage 306 is somewhat modified from that in the firstembodiment.

Note that, similarly to FIG. 9, FIG. 10 is a sectional view taken alongthe line B—B in FIG. 7.

In the second embodiment, as shown in FIG. 10, a partition 404 extendingin the lengthwise direction of the sub-passage 306 is formed in thesub-passage 306 for the purpose of further reducing the flow speed ofthe fluid in the state of the forward flow at the location where thethermally sensitive resistor 2 is disposed.

Additionally, the partition 404 has a taper 405 formed so as to define atubular path gradually widening toward the downstream side of thesub-passage 306. The presence of the taper 405 contributes to increasingthe amount of the backward flow flowing toward the side near thethermally sensitive resistor 2.

The sub-passage 306 thus constructed can provide similar advantages tothose obtained with the first embodiment. In addition, since the secondembodiment produces a larger difference in temperature of the thermallysensitive resistor 2 between the state of the forward flow and the stateof the backward flow than that in the first embodiment, a larger minusshift is obtained in the state of the backward flow and the plus errorcaused by the backward flow can be further reduced.

FIG. 11 is a schematic sectional view of a sub-passage 306 in a heatingresistor flow rate measuring instrument according to a third embodimentof the present invention. The other construction than the sub-passage isthe same as that of the above-described first embodiment, and henceillustration and a detailed description thereof are omitted here.

In the third embodiment of the present invention, as in the secondembodiment, the configuration of the sub-passage 306 is somewhatmodified from that in the first embodiment.

As shown in FIG. 11, the sub-passage 306 is formed such that it providesa simple tubular path 407 in the upstream side, abruptly narrows at thedownstream end of the tubular path 407, and then provides a tubular path406 gradually widening toward the downstream side. The thermallysensitive resistor 2 is disposed at the downstream end of the tubularpath 407 to lie on an extension of the center axis of the graduallywidening tubular path 406.

With the sub-passage 306 according to the third embodiment, in the stateof the forward flow, the resistance against passage of the fluidincreases and the flow rate of the incoming fluid reducescorrespondingly. In the state of the backward flow, the backward flowflows as a jet into the simple tubular path 406 from the graduallywidening tubular path 407, and therefore the effect of cooling thethermally sensitive resistor 2 is increased.

The third embodiment can also provide similar advantages to thoseobtained with the first embodiment.

Next, a heating resistor flow rate measuring instrument according to afourth embodiment of the present invention will be described withreference to FIGS. 12 and 13. In this fourth embodiment, the thermallysensitive resistor 2 is heated in a different manner from that in thefirst embodiment. FIG. 12 is a cross-sectional view of an instrumentmodule with the main passage 4 omitted, and FIG. 13 is an externalappearance view, looking from the upstream side, of the instrumentmodule. Note that FIG. 12 corresponds to a section taken along the lineD—D in FIG. 13.

An overall construction of the fourth embodiment is the same as that ofthe first embodiment, and therefore the following description is made ofonly different points from the first embodiment.

The biggest difference between the first embodiment and the fourthembodiment is as follows. In the first embodiment, the thermallysensitive resistor 2 is of the self-heating type that an electriccurrent is applied to the thermally sensitive resistor 2 to heat it. Inthe fourth embodiment, however, the thermally sensitive resistor 2 isnot of the self-heating type, and a separate heater 20 is disposedupstream of the thermally sensitive resistor 2 so that the thermallysensitive resistor 2 is heated by the heater 20.

More specifically, in the fourth embodiment, a heat flow generated underheating by the heater 20 heats the thermally sensitive resistor 2 in thestate of the forward flow. On the other hand, in the state of thebackward flow, the thermally sensitive resistor 2 is not affected by thetemperature of the heater 20 and is cooled so as to approach the fluidtemperature. Therefore, the temperature of the thermally sensitiveresistor 2 differs between the state of the forward flow and the stateof the backward flow.

In the fourth embodiment, the sub-passage 306 can be formed as a simpletubular path, and a flow restrictor or the like is not required.Further, the temperature to which the thermally sensitive resistor 2 isheated in the state of the forward flow can be adjusted depending on thespacing between the heater 20 and the thermally sensitive resistor 2 andon an extent by which they overlap with each other.

The fourth embodiment can also provide similar advantages to thoseobtained with the first embodiment.

While the heating resistor 1 and the thermally sensitive resistor 2 aredisposed in the sub-passage 3 in the embodiments described above, thelocations where the heating resistor 1 and the thermally sensitiveresistor 2 are disposed are not limited to positions in the sub-passage.

Specifically, the heating resistor 1 may be disposed at a first locationwithin the main passage 4 where the heating resistor 1 is exposed to thefluid flowing in one direction within the main passage 4 in a largeramount than the fluid flowing in a direction opposite to the onedirection, while the thermally sensitive resistor 2 may be disposed at asecond location within the main passage 4 where the thermally sensitiveresistor 2 is exposed to the fluid flowing in the opposite directionwithin the main passage 4 in a larger amount than the fluid flowing inthe one direction within the main passage 4.

Also, the first sub-passage may have a first opening that facessubstantially perpendicular to the fluid flowing in the forwarddirection (one direction), and a second opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction and has asmaller opening area than the first opening. The second sub-passage mayhave a third opening that faces substantially perpendicular to the fluidflowing in the forward direction, and a fourth opening that facessubstantially perpendicular to the fluid flowing in the oppositedirection and has a larger opening area than the third opening. Then,the heating resistor 1 may be disposed in the first sub-passage and thethermally sensitive resistor 2 may be disposed in the secondsub-passage.

Another modification may be constructed as follows. A first heatingresistor radiating a larger amount of heat in the state of the forwardflow than in the state of the backward flow and a second heatingresistor radiating a larger amount of heat in the state of the backwardflow than in the state of the forward flow are both disposed in the mainpassage 4, and a bridge circuit including the first heating resistor andthe second heating resistor is formed. Then, an applied electric currentis controlled so that the first heating resistor is held at atemperature higher than the second heating resistor by a certaintemperature. The flow rate of the fluid passing through the main passage4 is measured based on the amount of heat radiated from the firstheating resistor.

INDUSTRIAL APPLICABILITY

According to the present invention, in the heating resistor flow ratemeasuring instrument, when the fluid flows in the forward direction, theheating resistor 1 outputs a value depending on the flow rate of thefluid. When the fluid flows in the opposite direction, the thermallysensitive resistor 2 is cooled to a temperature lower than thatresulting when the fluid flows in the forward direction, and an electriccurrent flowing through the heating resistor 1 is decreased. As aresult, even when a pulsation occurs in the fluid, a precise flow ratecan be measured because not only a plus effect caused by the backwardflow is eliminated, but also a value corresponding to the backward flowis subtracted.

It is hence possible to precisely detect the flow rate in the forwarddirection even for a fluid that may cause a pulsation accompanying abackward flow, such as intake air of an automobile engine.

Particularly, in measurement of the flow rate of intake air of anautomobile engine, a large plus error resulting when a pulsationaccompanying a backward flow occurs near a throttle fully-opened strokein a specific range of revolution speed can be eliminated and fuelcontrol precisely responsive to operation conditions can be achieved.

1. A heating resistor flow rate measuring instrument comprising aheating resistor and a thermally sensitive resistor both disposed in amain passage, and measuring the flow rate of a fluid passing throughsaid main passage, said instrument including: a first sub-passageexposed to the fluid flowing in one direction within said main passagein a larger amount than the fluid flowing in a direction opposite to theone direction; and a second sub-passage exposed to the fluid flowing inthe opposite direction within said main passage in a larger amount thanthe fluid flowing in the one direction, wherein said heating resistor isdisposed at said first sub-passage, said thermally sensitive resistor isdisposed at said second sub-passage, the flow rate of the fluid passingthrough said main passage is measured based on amounts of heat radiatedfrom said heating resistor and said thermally sensitive resistor, andthe shape of said first sub-passage is different from that of saidsecond sub-passage.
 2. The heating resistor flow rate measuringinstrument according to claim 1, wherein said heating resistor is heatedto be higher than a fluid temperature in said main passage by a firstpredetermined temperature, and said thermally sensitive resistor isheated to be higher than the fluid temperature in said main passage by asecond predetermined temperature.
 3. The heating resistor flow ratemeasuring instrument according to claim 1 or 2, wherein said firstsub-passage has a first opening that faces substantially perpendicularto the fluid flowing in the one direction, and a second opening thatfaces substantially parallel to the fluid flowing in the oppositedirection, and said second sub-passage has a third opening that facessubstantially perpendicular to the fluid flowing in the oppositedirection, and a fourth opening that faces substantially parallel to thefluid flowing in the one direction.
 4. The heating resistor flow ratemeasuring instrument according to claim 1 or 2, wherein said firstsub-passage has a first opening that faces substantially perpendicularto the fluid flowing in the one direction, and a second opening thatfaces substantially parallel to the fluid flowing in the oppositedirection, and a wall portion having a surface substantiallyperpendicular to the lengthwise direction of said main passage is formedin the one-direction side of said second sub-passage.
 5. The heatingresistor flow rate measuring instrument according to claim 1 or 2,wherein said first location is provided by a first sub-passage having afirst opening that faces substantially perpendicular to the fluidflowing in the one direction, and a second opening that facessubstantially perpendicular to the fluid flowing in the oppositedirection and has a smaller opening area than said first opening, andsaid second location is provided by a second sub-passage having a thirdopening that faces substantially perpendicular to the fluid flowing inthe one direction, and a fourth opening that faces substantiallyperpendicular to the fluid flowing in the opposite direction and has alarger opening area than said third opening.
 6. The heating resistorflow rate measuring instrument according to claim 1 or 2, wherein saidthermally sensitive resistor is heated to a temperature 20° C.–40° C.higher than the fluid temperature in said main passage.
 7. The heatingresistor flow rate measuring instrument according to claim 1, whereinsaid first sub-passage includes a curved portion.
 8. The heatingresistor flow rate measuring instrument according to claim 1, wherein apartition extending in the lengthwise direction of the secondsub-passage is formed in the second sub-passage.
 9. The heating resistorflow rate measuring instrument according to claim 8, wherein saidpartition has a taper portion which defines a tubular path graduallywidening toward the downstream side of the second sub-passage.
 10. Theheating resistor flow rate measuring instrument according to claim 1,wherein said second sub-passage includes a simple tubular path in theupstream side, and a tubular path gradually widening toward thedownstream side, the downstream end of said simple tubular pathnarrowing abruptly.
 11. The heating resistor flow rate measuringinstrument according to claim 1, wherein a projection-shaped flowresistor is formed in the tubular path of said second sub-passage andthe thermally sensitive resister is arranged in a position shielded bythe flow sensor when viewed from the upstream side.
 12. A heatingresistor flow rate measuring instrument for measuring the flow rate of afluid passing through a passage, said instrument comprising: a firstheating resistor radiating a larger amount of heat to the fluid flowingin one direction within said passage than to the fluid flowing in adirection opposite to the one direction; and a second heating resistorradiating a larger amount of heat to the fluid flowing in the oppositedirection than to the fluid flowing in the one direction; wherein abridge circuit including said first heating resistor and said secondheating resistor is formed, said first heating resistor is heated to behigher than said second heating resistor by a predetermined temperature,and the flow rate of the fluid passing through said passage is measuredbased on the amount of heat radiated from said first heating resistor.13. A heating resistor flow rate measuring instrument comprising aheating resistor and a thermally sensitive resistor both disposed in amain passage, and measuring the flow rate of a fluid passing throughsaid main passage, wherein: said instrument includes athermally-sensitive-resistor arrangement sub-passage in which saidthermally sensitive resistor is disposed and radiates a larger amount ofheat when the fluid flows in a direction opposite to one directionwithin said main passage than when the fluid flows in the one direction;said heating resistor is heated to be higher than a fluid temperature insaid passage by a first predetermined temperature; said thermallysensitive resistor is heated to be higher than the fluid temperature insaid passage by a second predetermined temperature; and the flow rate ofthe fluid passing through said passage is measured based on amounts ofheat radiated from said heating resistor and said thermally sensitiveresistor.
 14. The heating resistor flow rate measuring instrumentaccording to claim 13, wherein said heating resistor is disposed at alocation in which said heating resistor radiates a larger amount of heatwhen the fluid flows in the one direction within said main passage thanwhen the fluid flows in the opposite direction.